1. Field of the Invention
The present invention relates generally to poled electro-optic devices, and particularly to glass poled electro-optic devices.
2. Technical Background
Poling glass fibers in general and fused silica optical fibers in particular, is known to make the fibers electro-optically "active" or into an electro-optically responsive medium.
One application of an electro-optically responsive medium, such as an organic non-linear optical polymer or an inorganic waveguide, is its use as a switch. The polarization properties of integrated optical switches, modulators, filters, and other applications, are important in determining their utility in the fiber communication system. These optical devices must perform efficient and complete switching of light, without regard to the light's state of polarization because linearly polarized light coupled into single-mode circular fibers suffers a rapid conversion to other polarization states. Light coupled from a fiber therefore usually possesses an unknown elliptical polarization, and both transverse horizontal electric (TE) and transverse vertical magnetic (TM) modes will be excited in the integrated optical circuit.
One known polarization-independent waveguide device is constructed with an inorganic waveguide channel such as with crystalline LiNbO.sub.3 (Niobate). However, there are certain disadvantages in the use of Niobate or other inorganic crystal because of the limitation of the input optical power and operational wavelength due to the inherent photorefractive effect.
An organic polarization-insensitive linear waveguide electro-optic waveguide phase modulator has been developed to overcome the bandwidth limitations of conventional inorganic electro-optic materials. In this linear organic waveguide structure, the optical path has two sets of electrodes, which apply a horizontal electric field and a vertical electric field, respectively, to a nonlinear optically responsive polymeric medium in the optical path. The polymer medium section between each set of electrodes has a noncentro-symmetric molecular orientation parallel to the respective electric fields.
Another application of electro-optically responsive medium and desired result of the poled fiber is to be able to induce an electro-optic coefficient as large as possible or at least larger than those obtained in Lithium niobate crystals, the commonly used electro-optically responsive modulator for advanced optoelectronic systems in order to reduce packaging costs. For a high-speed fiber communication system, the output of a diode laser is often coupled into a single-mode optical fiber. The fiber, alternately referred to as an optical waveguide as the more general term, is then coupled to a Lithium niobate waveguide modulator whose output is again coupled into a fiber. Discrete optical components, such as graded-index lenses, micro-lenses or other beam-shapers are needed at each coupling node to adapt or match the different mode profiles and spatial extents of the diode laser and modulator waveguide modes to the fiber mode. Tight tolerances, in the range of micrometers, are required to minimize coupling losses. If a modulator could be simply produced that was integrated into the fiber that can simply be spliced onto the laser fiber, the manufacturing and packaging costs associated with these high-speed fiber communication systems would be reduced.
Recently, fiber has been induced with a significant improvement of the effective electro-optic coefficient by the use of an ultraviolet beam along with an applied electric field close to the core of the single mode fiber to produce the poling in contrast to the use of high temperatures under an applied electric field, and by the provision for two wire electrodes internal to the fiber to increase resistance to breakdown during poling and to provide a better overlap between the nonlinearity and the optical mode volume. Small lengths of fiber (about 10 cm) is drawn from a preform with two large holes close to the core for thin electrode wires that are to be inserted or threaded following the fiber drawing. This wire insertion is a difficult manufacturing step. To avoid breakdown, one short wire is inserted from each end of the fiber.
Because the holes in the fiber need to be significantly larger than the electrode diameter for the "threading" process, this dimensional control is difficult, resulting in significant variation in the distance between the electrodes and core and between the electrodes. Hence, a longitudinal non-uniformity of the applied poling field and of the electro-optic coefficient can result.
One low-cost alternative fabrication technique for an electro-optically active fiber segment deposits a dielectric isolation structure surrounding an etched "D" shaped fiber that is glued to a flat substrate with a conductive surface for forming a first electrode to allow high field poling while allowing the ends of the fiber to extend beyond the substrate for later splicing with additional fiber sections. The fiber/dielectric structure is polished to provide a planar surface on the side opposite the substrate. A second metal layer is deposited on the planar surface over the fiber to form a second electrode. Such a sample was poled and placed in the measurement arm of a Mach-Zehnder interferometer. As expected, the resultant phase shift signal was found to be sensitive to the polarization of input laser light. Consistent with the symmetry arrangement of the Mach-Zehnder interferometer arms, the ratio of the signal from TE mode to one from TM mode was about 2.4:1 which implies that the electro-optic coefficient ratios dropped along the traveled lengths of the Mach-Zehnder arms due to the mode changes.
Hence, there is a need for an improved electro-optically active fiber that can be simply spliced with other fiber components to significantly reduce the manufacturing costs.